Composition of organic-N and 15N-labelling uniformity in alder leaves

The composition of organic-N in alder leaves (Alnus cordata and A. incana) fertilized with 15N-labelled KNO3 was studied by a two-step hydrolysis with 1n and 3n hydrochloric acid. The distribution of organic-N (NH4+-N, amide-N, hexosamine-N, amino acid-N, hydrolyzable unidentified-N and non-hydrolyzable-N) did not exhibit significant differences between the two species. Of the total N 80% was hydrolyzable, with 55–61% being extracted in the first hydrolysis (HCl 1n × 3 h) and 19–24% in the second (HCl 3n × 3 h). The major fraction was amino acid-N (41%), followed by hydrolyzable unidentified-N (31%), amide-N (5%), hexosamine-N (1–2%) and ammonium-N (<1%). The isotopic abundance of 15N in some fractions differed from 15N values of total leaf organic-N (P < 0.05 to P < 0.01), suggesting that the leaf material was not uniformly labelled. Nevertheless, these differences were less than 3% in the major fractions (amino acids, hydrolyzable unidentified-N and non-hydrolyzable organic-N), which accounted for more than 92% of leaf N. This last result, and the fact that in both species 15N distribution did not differ significantly from 14N distribution, suggests that labelled alder leaves obtained by this method are acceptable for decomposition studies or for leaf N availability experiments.